1. Field of the Invention
The invention relates to control of drilling apparatus in well drilling. More particularly, the invention relates to a method for determining the location and attitude of a bottom hole assembly (BHA) and the diameter of the borehole at the location of the measurement in a BHA.
2. Prior Art
The displacement of a bottom hole assembly (or BHA) with respect to the borehole center at the location of the measurement sub in a BHA is useful for avoiding tool failure. Similarly, the knowledge of the size of the borehole is useful for 1) deviation control and 2) corrections to resistivity and neutron tools results. More specifically, drilling with a slightly bent drill collar can cause mild whirling to violent lateral vibrations. The cause of misalignment of the tools axially that might exacerbate whirling problems are bending or sag of a drill collar which may be due to (or caused by) an initial bend or curvature in the collar, collar sag from gravitational forces or from unbalanced tools or sub-sections of the bottom hole assembly. The bow of the tool caused by the bend in combination with rotation about the borehole centerline and its own center, can cause the collar to whirl in a complicated manner. Such whirl causes chaotic displacements, collar-borehole wall impacts and friction at the collar-wall interface and each of these can extend from inconsequential to damaging.
Whirling can be ultimately destructive when the rotation rate of the assembly equals the natural frequency of the bottom hole assembly in bending. Surface abrasion of collars and fatigue failures can also readily occur and are a direct result of whirling.
Surface abrasion is normally caused by forward synchronous whirl of a high amplitude and usually affects the same side of the drill collar which is continually in contact with the borehole wall. Clearly wear in the affected area is enhanced and can cause both unbalance problems and direct failure of the collar.
Another possible complex movement with destructive consequences, if left unchecked, is backward whirl. Backward whirl causes fatigue failures of the bottom hole assembly components by inducing high frequency stress cycles therein. Backward whirl is the whirling of a drill collar, without slipping along the borehole wall, with the center of the collar rotating in a direction opposite to the imposed direction of collar rotation. The frequency with which the collar center rotates about the borehole center is generally much higher than the rotation rate of the collar. Determining that the collar is undergoing such a condition is obviously a precursor to alleviating the problem. Unfortunately, the prior art provides no reliable method of so determining and the condition is therefore left unchecked. Information gatherable with the present invention regarding whether a collar is undergoing forward or backward whirl, with or without slip is invaluable for avoiding failures since adjustments could be made prior to failure. Therefore if one knows what these conditions are at any given time, or preferably continuously, adjustments can be made to increase drilling efficiency and reduce the chances of breakage. For example, information regarding conditions being experienced while drilling might indicate a change in the drilling procedure such as a faster or slower drill speed or a different mud consistency or composure. Where these adjustments are made in a timely manner, difficulties can be avoided whereas breakage or other tool failure might be experienced if the drilling personnel is not aware of the BHA condition downhole.
Information such as that identified above has been sought for a number of years as it is known to the oil industry that knowledge of downhole conditions are one of the ways to ensure successful drilling and ultimate production. For this reason, some methods have been developed to provide information at the surface so that decisions can be made. Information reaching the surface quickly enough to allow for real time correction is particularly desirable.
One prior art method is disclosed in U.S. Pat. No. 5,313,829 to Pasley et al which is directed to determining bottom hole assembly lateral bending vibrations associated with elongated drill strings. The method incorporates a mechanical model which converts lateral vibrations to longitudinal and torsional vibrations measurable at the surface. The model simulates the condition of BHA whirling, incorporates the effects of bending due to whirl forces, and determines the shape of the bottom hole assembly during a particular vibration mode.
Another prior art method for analyzing drillstring vibration is disclosed in U.S. Pat. No. 5,321,981 to Macpherson. In this method, it is recognized that torsional vibration of a drillstring during drilling will lead to frequency modulation (FM) of the signal from a vibratory source. Such vibratory sources include for example, the drill bit. The frequency modulation of the signal results, in the frequency domain, in sidebands around the detected excitation frequency.
The method disclosed in the Macpherson patent employs the sidebands to improve drillstring and drilling performance. More particularly, the sidebands are used to discriminate between downhole and surface vibrational sources caused by torsionally induced frequency modulation. The sidebands are also used to determine the rotary speed of bottom hole assembly components. These are determined as a function of the excitation frequency, the frequency of torsional oscillation and the modulation index. Based upon all of the information gatherable using this method, the parameters of the drillstring or the drilling operation can be altered to optimize either the drillstring performance or the drilling operation. Unfortunately, although the prior art is effective for the intended purpose, it does not cure all of the problems experienced in relation to whirl.
A related problem in drilling with respect to which information would be helpful is the fact that as a result of some of the complex rotation discussed above and other reasons, the borehole becomes enlarged. A large hole causes the drill bit to deviate from its intended path. In addition, neutron and resistivity tool measurements, for example, will provide falsely interpreted results based upon such measurements. The prior art has not attempted to provide information directed to help alleviate this occurrence.
While the foregoing methods are workable for their intended purposes, other methods having specific benefits are still sought.